The fraction of inspired oxygen (FiO2) represents the concentration of oxygen a person is inhaling, expressed as a percentage of the total gas volume. Normal air has an FiO2 of approximately 21%. Supplemental oxygen is prescribed when a person’s blood oxygen levels are too low, a condition medically termed hypoxemia. The oxygen flow rate is measured in liters per minute (LPM). Determining the exact FiO2 delivered by a specific LPM flow rate, such as 6 LPM, is complex. The concentration received is an estimate dependent on how the oxygen interacts with surrounding air and the patient’s breathing pattern.
Understanding Oxygen Concentration and Flow Rate
The key difference between FiO2 and flow rate is that FiO2 describes the percentage of oxygen in the inspired breath, while LPM describes the volume of gas leaving the delivery device. Oxygen sources deliver gas that is nearly 100% oxygen. When this pure oxygen is delivered through a low-flow device like a nasal cannula, it is diluted by a process called air entrainment. The cannula delivers a high-concentration jet of oxygen into the nasal passages. As the patient inhales, this oxygen mixes with the surrounding room air, which is 21% oxygen. This mixing creates a variable, lower concentration that the patient breathes into their lungs. Since the nasal cannula is a low-flow system, it cannot meet the patient’s total air requirement, meaning the patient must always draw in room air to complete the breath.
Calculating the Estimated FiO2 Delivered by a Nasal Cannula
To determine the approximate FiO2 delivered by a low-flow nasal cannula, clinicians rely on the widely accepted 4% rule. This model assumes a patient has a normal, stable breathing pattern, which allows for a consistent calculation. The rule states that for every 1 LPM increase in oxygen flow rate, the FiO2 increases by roughly 4% above the baseline 21% concentration of room air.
The calculation begins with the 21% ambient air concentration. When the flow is set to 1 LPM, the FiO2 is estimated to be 25% (21% + 4%). This linear progression continues for each subsequent liter, allowing for a quick estimation of the oxygen concentration. For example, 2 LPM is 29% FiO2, while 3 LPM delivers approximately 33% FiO2.
Applying this formula to 6 LPM, the 21% baseline is added to the product of 6 LPM and the 4% increase per liter. This calculation results in an estimated FiO2 of 45%. However, 44% is commonly cited as the maximum achievable FiO2 for a standard nasal cannula. Flow rates above 6 LPM are generally discouraged due to poor efficiency and the potential for nasal discomfort.
Factors That Change the Actual Oxygen Received
The estimated 44% FiO2 at 6 LPM is a theoretical maximum rarely achieved in a clinical setting because patient physiology significantly impacts the actual inhaled concentration. The calculation assumes a patient with a steady, moderate breathing pattern, which is often not the case when supplemental oxygen is required. The precise FiO2 delivered varies with the patient’s individual respiratory mechanics, which affect the amount of room air entrained with each breath.
Respiratory Rate
One major physiological variable is the patient’s respiratory rate, or how many breaths they take per minute. When a patient breathes faster, the amount of time available for the supplemental oxygen to fill the nasal passages and upper airway is decreased. This reduced filling time means less oxygen is available during inspiration, leading to a greater reliance on entrained room air and a significant drop in the actual FiO2 delivered.
Tidal Volume (Depth of Breath)
The depth of a patient’s breath, known as tidal volume, also plays a substantial role in diluting the estimated FiO2. A patient taking deeper breaths requires a larger volume of gas to fill their lungs. Because the nasal cannula is a low-flow device, a deep breath will quickly consume the small amount of concentrated oxygen in the upper airway, forcing the patient to draw in a much larger volume of 21% room air to complete the inhalation.
Mouth Breathing
Furthermore, breathing through the mouth rather than the nose can drastically reduce the effective FiO2. A nasal cannula delivers oxygen directly into the nasopharynx, which acts as a small reservoir of concentrated oxygen. If a patient is primarily breathing through their mouth, they bypass this reservoir, drawing air directly from the atmosphere and diluting the oxygen concentration before it ever reaches the lungs.